This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Natural resources, such as oil and gas, are a common source of fuel for a variety of applications, such as to heat homes, to power vehicles, and to generate electrical power, to name just a few. Once a desired resource is discovered below the surface of the earth, drilling and production systems are typically employed to access, extract, and otherwise harvest the desired resource. These systems may be located onshore or offshore depending on the location of the desired resource. When a resource is located offshore (e.g., below a body of water), a subsea production system may be utilized to extract the resource. Such subsea production systems may include components located on a surface vessel, such as a rig or platform, as well as components located remotely from the surface vessel at a subsea location, typically at or near a subterranean formation (e.g., a well) at which the resource is located. For example, a subsea production system may utilize one or more subsea wellhead assemblies and Christmas trees for controlling the flow of a resource into or out of a well.
Additionally, a subsea production system may utilize one or more subsea loads driven by AC power, such as a pump, a motor, or a compressor, for facilitating the extraction of resources from the well. For instance, as a resource is gradually extracted from a well over time, the innate pressure within the well may decrease. Thus, at some point during the life of the well, a subsea pump may be utilized to facilitate extraction of the resource from the well to the surface vessel. Such subsea loads (e.g., pumps, compressors, and motors) are generally powered using AC power, typically on the order of hundreds of kilowatts or even megawatts, supplied by an AC power supply located on the surface vessel. Typically, a variable frequency drive may be provided in conjunction with the AC power supply to provide for operation of subsea loads at variable speeds. For example, variable frequency drive may include an inverter that provides AC power to the subsea load at controllable frequencies, thus providing for adjustable control of the subsea load. As can be appreciated, this may allow for subsea pumps and compressors to be started at lower frequencies and then gradually ramped up to a desired operating speed.
In conventional subsea production systems, the variable frequency drive is typically located either on the surface vessel (in the general proximity of the surface AC power supply) or subsea (in the general proximity of the subsea load). For instance, when a subsea load is located relatively close to a surface vessel (e.g., approximately 15 kilometers or less), the variable frequency drive may be located on the surface vessel, generally in close proximity to an AC power supply. In operation, the AC power output provided by the surface variable frequency drive is transmitted to a subsea load using one or more power conductors enclosed within an umbilical. By way of example, where three-phase AC power is being transmitted from the surface variable frequency drive to the subsea load, the umbilical may include three AC power lines for transmitting the three-phase AC power (e.g., including 15 Hz, 30 Hz, and 60 Hz AC power).
When the subsea load is located farther away from the surface vessel (e.g., greater than 15 kilometers), it may not be desirable to have the variable frequency drive located on the surface vessel, due at least partially to undesirable harmonics and reflective waveforms that may result due to the nature of transmitting AC power over long distances. In such applications, it may be practical to utilize a variable frequency drive that is located subsea and away from the surface vessel (e.g., located generally in the proximity of the subsea load). In this configuration, AC power from an AC power supply on the surface vessel may be transmitted using the above-mentioned umbilical to the subsea variable frequency drive to provide power for driving the subsea load at variable speeds.
Unfortunately, the transmission of AC power, particularly over long step-out distances, is not always efficient. In an effort to increase the efficiency of AC power transmission, one technique that has been utilized is to raise the AC voltage being transmitted via an AC power conductor using subsea and/or surface transformer components. However, such components are generally costly and may add to the overall costs of resource extraction. Another technique that has been utilized for improving AC power transmission efficiency is to transmit lower frequency AC power. However, even using such measures, the transmission of AC power at a relatively low frequency of 15 Hz may still result in a decrease of over 20% efficiency at a step-out distance of approximately 200 kilometers. Additionally, the relatively high cost of providing an umbilical having sufficient core size to transmit AC power (particularly multi-phase AC power) over long distances is often burdensome and adds to the overall cost of resource extraction. Further, the transmission of AC power over long distances may additionally result in potentially undesirable harmonics and reflective waveforms being generated near sensitive subsea electronic equipment. Still further, in applications where a variable frequency drive is located subsea for the operation of AC powered subsea loads, the servicing, repair, and/or maintenance of the variable frequency drive may be impractical and/or difficult.
In light of the above-mentioned drawbacks, among others, it may be desirable to provide a more efficient technique for powering and controlling AC powered subsea loads.